Optical amplifiers utilized in optical communication apparatuses etc. are adapted to implement optical amplification on optical level of optical input signals, and to output the amplified optical signals. Generally, an output end of the optical amplifier is configured as a coupling connector for an optical fiber, and is connected to other device through the optical fiber.
In the case where the connector is failure in connection, the optical signal output from the optical amplifier leaks from the output end toward free space. Since the optical signal leaking from the optical amplifier possesses high power, the leak light may trigger a rise in temperature of objects around the optical amplifier or may cause dangerous to human beings. If the leak light hits human eyes, he/she may be blinded.
For the above reasons, the optical amplifier is adapted to activate an eye protective function to automatically reduce the output level of the optical amplifier in the case where the output end as the connector is failure in connection. Upon occurrence of the failure in connection, the optical fiber of output side becomes an open end. A part of the light is reflected at the open end, and then returns to the optical amplifier as reflected-light. The eye protective function is adapted such that the detector is provided to detect the reflected-light input to the optical amplifier. The eye protective function is further adapted to determine the occurrence of failure in connection when the detector detects reflected-light, and to reduce an amount of optical amplification of the optical amplifier (Ref. e.g. Patent Literatures 1 and 2).
Besides the reflected-light caused by trouble of a transmission line such as the failure in connection discussed in above, optical feedback may occur in some cases. The optical feedback comes from a device connected to the optical amplifier toward the output end of this optical amplifier. Although causes of the optical feedback depend on a kind of the device, one of exemplary causes is assumed by a multistage connection of optical amplifiers. In such the multistage connection, leak light of excitation light used for optical amplification at a later stage of the multistage may return to an earlier stage. The detector at the earlier stage detects optical level of reflected-light to which optical feedback is added. A threshold is therefore set to the optical amplifier in order to distinguish whether the light from the output end is reflected-light caused by trouble of a transmission line or by optical feedback which constantly comes from the device. This optical amplifier determines the occurrence of trouble of the transmission line in the case where the detected optical level exceeds the threshold, and then activates the eye protection function to reduce the amount of optical amplification of the optical amplifier.